Chain cable

ABSTRACT

A chain cable in which each link is characterised by being formed from a bar .[.bent into a.]. .Iadd.of .Iaddend.U-shape bridged at the extremities by a cylindrical pin of length substantially equal to its diameter which mates under load with the U-shaped bar of a next adjacent link to give a substantial bearing area between links so that bearing failure may be eliminated as a first-occurring manner of failure of the chain cable when it is subjected to a tensile load.

The present invention relates to chain cable, for example: chain cableas used for mooring ships, floating drilling rigs, barges, and the like.

In the past, heavy chain cables, such as used for mooring ships, havebeen fabricated from round section bar forming links wherein each linkhas had three orthogonal modes of rotational freedom with respect to anext adjacent mating link. Generally, each link is formed from roundsection bar bent and welded to form a closed oval loop which may bebraced across a minor diameter by a strut known as a "stud." A joinedsequence of such links in standard lengths coupled by special joininglinks comprises conventional chain cable.

A disadvantage of such chain cable lies in the fact that, owing to thelinking of curved round-section bars, only point contact initiallyoccurs between links bearing one against the other when the cable is intension under the action of a load. This point contact gives rise toenormous bearing pressure between links, even under relatively lightloading, which causes the metal of the link to fail locally in bearingand be extruded away form the high pressure locality until the bearingarea has increased sufficiently to carry the applied load. As thetensile load in the cable increases, bearing failure occurs again andmetal is extruded from the bearing zone progressively until thecross-sectional area of the round section bar of the link is so reducedin the region of bearing failure that terminal shear failure occurs andthe cable parts suddenly.

Failure in bearing thus constitutes the first-occurring manner offailure in present day stud-link chain cable and, coupled withconsequentially precipitated failure in shear, commonly results in chaincable failing at less than half the tensile load capacity of thematerial from which the cable is formed.

An object of the present invention is to provide a chain cable whichdoes not have bearing failure as the first-occurring manner of failureand which provides high strength relative to the weight of material inthe cable.

According to the present invention, a chain cable comprises a pluralityof links, joined together in sequence, characterized in that each linkincludes a bar member .[.bent into a.]. .Iadd.of .Iaddend.U-shape and acylindrical pin member bridging the inside surfaces of the extremitiesof the bar member and attached thereto such that under load the curvedinside surface of the U-shaped bent bar member of a next adjacent linkmates against a surface of the pin member whereby the pin member isrestrained to have only one mode of rotational freedom relative to thesaid next adjacent link, the radius of curvature of the cylindricalmating surface of the pin member being substantially equal to half thedistance separating the inside surfaces of the extremities of the barmember.

Preferably the cross-sectional area of the pin member is at least equalto the least cross-sectional area of a limb of the U-shaped bar memberso that the shear stress in the pin member does not exceed the tensilestress in the bar member.

Preferably the bar member is of rectangular cross-section.

Preferably the pin member is a hollow circular cylinder.

Preferably where the pin member is a hollow circular cylinder, theinternal diameter of the pin member is substantially equal to 0.707times its external diameter.

Preferably the overall length of a link of the chain cable lies in therange 4 to 6.5 times the external diameter of the link pin.

Embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawings in which:

FIG. 1 shows a sectioned perspective view of a three-link sample of thechain cable;

FIG. 2 shows a sectional side elevation of a link of the chain cablehaving a hollow pin;

FIG. 3 shows a plan view of the link shown in FIG. 2;

FIG. 4 shows an exploded perspective view of a special joining link forjoining lengths of chain cable formed of links as shown in FIGS. 2 and3.

Referring to FIG. 1, a chain cable includes a number of similar linkseach comprising a length of rectangular cross-section bar 1 bent into aU-shape with parallel limbs joined at their extremities by a circularcylindrical pin 2. The radius of curvature of the inner surface of bar 1remote from pin 2 equals or just slightly exceeds half the width of bar1 measured parallel to the axis of curvature. Pin 2 is welded inposition and has a diameter equal to its length which, in turn, is equalto the distance separating the opposed inner surfaces of the extremitiesof bar 1.

Each link encircles the pin of the next adjacent link such that, whenunder tension, the curved inner surfaces of bars 1 mate against thesurfaces of pins 2 whereby each link is restrained to one mode ofrotational freedom relative to a next adjacent link and whereby abearing area equal to the square of the pin diameter is obtained whenmeasured as projected on a plane orthogonal to the axis of the chaincable.

The cross-sectional area of pin 2 measured across its axis is at least10 per cent greater than the cross-sectional area of a limb of bar 1 andthe said width of bar 1 so that the shear stress in the pin is alwaysappreciably less than the tensile stress in the bar for a given tensionof the cable and is just sufficiently less than the diameter and lengthof pin 2 to allow bar 1 to swing freely within the limbs of thecorresponding bar 1 of the next adjacent link. The overall length ofeach link lies typically in the range 4 to 6.5 times the diameter of pin2.

In a length of chain cable formed by a number of joined linksconstructed as described, each link is angularly orientated at rightangles to the previous link. The cable thus can form curves in twoplanes at right angles to each other but, owing to the constraint ofeach link pin to one mode of rotational freedom, the cable remains rigidin torsion.

Referring to FIGS. 2 and 3, in a preferred embodiment of the presentinvention, pin 2 is shown hollow and having an internal bore co-axialwith the pin axis. The internal diameter of the hollow pin 2 is equal to0.707 times the external diameter so that the cross-sectional area ofthe bore equals the cross-sectional area of the material of the hollowpin.

Referring to FIG. 4, a modification of the preferred embodiment of FIGS.2 and 3 is shown which forms a joining link for joining lengths of cabletogether and which comprises a U-shaped bar 3, a cylindrical pin 4, atubular sleeve 5, and a taper pin 6. The U-shaped bar 3 is similar tothe previous bar 1 but has an increased depth of section at theextremities of its limbs. These deepened extremities carry co-axial boreholes 7 through which may be fitted the cylindrical pin 4. The tubularsleeve 5 is dimensionally identical to the hollow link pin 2 shown inFIGS. 2 and 3 and is removably located in the link pin position of bar 3by passing pin 4 through sleeve 5 and bore holes 7. Pin 4 has across-sectional area equal to that of the bore of sleeve 5 and is lockedin position by a taper pin 6 locating in hole 8 in bar 3 and in hole 9in pin 4.

Joining of two cable lengths formed of hollow pin links is effected byassembling a joining link, as shown in FIG. 4, through an end link ofeach cable length. The joining link has equal strength with that of acommon hollow pin link so that no weak link is present in the cableformed from the two joined lengths of chain. Where twisting forces mustbe accommodated, a swivel piece can be included between lengths of thecable.

A chain cable, constructed as hereinbefore described, can provide aconsiderably higher tensile strength relative to the weight of materialin the cable than that of conventional cable with links formed fromround section bars.

I claim:
 1. A chain cable comprising a plurality of links, joinedtogether in sequence, characterised in that each link includes a uniformbar member .[.bent into a.]. .Iadd.of .Iaddend.U-shape and a cylindricalpin member bridging the inside surfaces of the extremities of the barmember and attached .Iadd.integrally .Iaddend.thereto .[.by welding.].such that under load the curved inside surface of the .[.bent.].U-shaped bar member of a next adjacent link mates against a surface ofthe pin member whereby the pin member is restrained to have only onemode of rotational freedom relative to the next adjacent link, theradius of curvature of the cylindrical mating surface of the pin memberbeing substantially equal to half the distance separating the insidesurfaces of the extremities of the bar member and the cross-sectionalarea of the material of the pin member being not less than 1.1 times thecross-sectional area of a limb of the uniform bar member.
 2. A chaincable, as claimed in claim 1, characterised in that the bar member ofeach link is of rectangular cross-section.
 3. A chain cable, as claimedin claim 1, characterized in that the pin member of each link is ahollow circular cylinder.
 4. A chain cable, as claimed in claim 3,characterised in that the hollow pin member of each link has an internalbore diameter substantially equal to 0.707 times its external diameter.5. A chain cable, as claimed in claim 1, characterized in that theoverall length of each link lies in the range 4 to 6.5 times theexternal diameter of the pin of each link.